diff --git a/apps/desktop/src/app/messaging/index.tsx b/apps/desktop/src/app/messaging/index.tsx
index 0a1cc8e7ff..1611dcf3c2 100644
--- a/apps/desktop/src/app/messaging/index.tsx
+++ b/apps/desktop/src/app/messaging/index.tsx
@@ -28,7 +28,7 @@ import { normalize } from '@/lib/text'
 import { cn } from '@/lib/utils'
 import { $changeEventsAvailable, $pairingChangeTick, $platformsChangeTick } from '@/store/live-sync'
 import { notify, notifyError } from '@/store/notifications'
-import { $settingsScopeOverride } from '@/store/settings-scope'
+import { $settingsRequestProfile } from '@/store/settings-scope'
 import { runGatewayRestart } from '@/store/system-actions'
 
 import { useRefreshHotkey } from '../hooks/use-refresh-hotkey'
@@ -128,10 +128,9 @@ function fieldCopy(field: MessagingEnvVarInfo, m: Translations['messaging']) {
 export function MessagingView({ setStatusbarItemGroup: _setStatusbarItemGroup, ...props }: MessagingViewProps) {
   const { t } = useI18n()
   const m = t.messaging
-  // Shared settings "Applies to" scope. At this layer `null` means "follow
-  // the active profile", while the API helpers use `undefined` for that
-  // contract; passing null would deliberately target the primary profile.
-  const scopeProfile = useStore($settingsScopeOverride) ?? undefined
+  // Shared settings "Applies to" scope, request-shaped (undefined → follow
+  // the active profile; the API helpers treat null as "target primary").
+  const scopeProfile = useStore($settingsRequestProfile)
   // Both save/toggle toasts offer the same one-click restart.
   const restartGatewayAction = { label: t.commandCenter.restartGateway, onClick: () => void runGatewayRestart() }
   const [platforms, setPlatforms] = useState<MessagingPlatformInfo[] | null>(null)
diff --git a/apps/desktop/src/app/settings/config-settings.tsx b/apps/desktop/src/app/settings/config-settings.tsx
index c44dd8b6a7..ee69a841fc 100644
--- a/apps/desktop/src/app/settings/config-settings.tsx
+++ b/apps/desktop/src/app/settings/config-settings.tsx
@@ -24,7 +24,7 @@ import { $keepAwake, setKeepAwake } from '@/store/keep-awake'
 import { notify, notifyError } from '@/store/notifications'
 import { normalizeProfileKey } from '@/store/profile'
 import { repoDiscoveryPolicyFromConfig, repoDiscoveryPolicySignature, scanAndRecordRepos } from '@/store/projects'
-import { $settingsScopeOverride } from '@/store/settings-scope'
+import { $settingsRequestProfile } from '@/store/settings-scope'
 import type { ConfigFieldSchema, HermesConfigRecord } from '@/types/hermes'
 
 import { hermesConfigCacheWriter, useHermesConfigRecord } from '../hooks/use-config-record'
@@ -58,7 +58,7 @@ export function ConfigSettings({
   // inner page per scope so every draft/seed/autosave ref resets wholesale
   // when the target profile changes — the same guarantee useOnProfileSwitch
   // provides for app-wide switches, without hand-clearing each piece.
-  const scopeProfile = useStore($settingsScopeOverride)
+  const scopeProfile = useStore($settingsRequestProfile)
 
   return (
     <ConfigSettingsInner
@@ -85,7 +85,7 @@ function ConfigSettingsInner({
   onMainModelChanged,
   importInputRef,
   scopeProfile
-}: ConfigSettingsProps & { scopeProfile: null | string }) {
+}: ConfigSettingsProps & { scopeProfile: string | undefined }) {
   const { t } = useI18n()
   const c = t.settings.config
   const keepAwake = useStore($keepAwake)
@@ -108,7 +108,7 @@ function ConfigSettingsInner({
     // consumer); suffixed only for an explicit scope override.
     queryKey:
       scopeProfile == null ? ['hermes-config-schema'] : ['hermes-config-schema', normalizeProfileKey(scopeProfile)],
-    queryFn: () => getHermesConfigSchema(scopeProfile ?? undefined),
+    queryFn: () => getHermesConfigSchema(scopeProfile),
     staleTime: 5 * 60 * 1000
   })
 
@@ -147,7 +147,7 @@ function ConfigSettingsInner({
   useEffect(() => {
     let cancelled = false
 
-    getElevenLabsVoices(scopeProfile ?? undefined)
+    getElevenLabsVoices(scopeProfile)
       .then(result => {
         if (cancelled || !result.available) {
           return
@@ -178,7 +178,7 @@ function ConfigSettingsInner({
     const t = window.setTimeout(() => {
       void (async () => {
         try {
-          const result = await saveHermesConfig(config, scopeProfile ?? undefined)
+          const result = await saveHermesConfig(config, scopeProfile)
 
           if (!result.ok) {
             throw new Error(c.autosaveFailed)
diff --git a/apps/desktop/src/app/settings/env-credentials.tsx b/apps/desktop/src/app/settings/env-credentials.tsx
index 0b4daf8e2c..490d08c066 100644
--- a/apps/desktop/src/app/settings/env-credentials.tsx
+++ b/apps/desktop/src/app/settings/env-credentials.tsx
@@ -43,8 +43,10 @@ export function SettingsCategoryHeading({ count, icon: Icon, title }: CategoryHe
 // credential pages (Providers, Keys) share one source of truth and one set of
 // mutation handlers instead of duplicating the plumbing. An optional `profile`
 // targets another profile's env store (the shared settings "Applies to"
-// scope); undefined/null keeps the app-wide active profile.
-export function useEnvCredentials(profile: null | string = null): UseEnvCredentials {
+// scope); undefined keeps the app-wide active profile. Request-shaped on
+// purpose: the API helpers treat an explicit `null` as "target the
+// primary/default backend", which is never what a settings page means.
+export function useEnvCredentials(profile?: string): UseEnvCredentials {
   const { t } = useI18n()
   const credentials = t.settings.credentials
   const toolsets = t.settings.toolsets
diff --git a/apps/desktop/src/app/settings/keys-settings.test.tsx b/apps/desktop/src/app/settings/keys-settings.test.tsx
index 87217b2e15..699fd0e4a7 100644
--- a/apps/desktop/src/app/settings/keys-settings.test.tsx
+++ b/apps/desktop/src/app/settings/keys-settings.test.tsx
@@ -12,7 +12,7 @@ stubResizeObserver()
 
 vi.mock('@/hermes', () => ({
   deleteEnvVar: vi.fn(),
-  getEnvVars: () => getEnvVars(),
+  getEnvVars: (profile?: null | string) => getEnvVars(profile),
   revealEnvVar: vi.fn(),
   setApiRequestProfile: () => undefined,
   setEnvVar: vi.fn()
@@ -54,6 +54,15 @@ function DeepLinkButton({ target }: { target: string }) {
 }
 
 describe('KeysSettings', () => {
+  it('fetches env vars for the active profile (undefined, never null) when unscoped', async () => {
+    // #90549 class: getEnvVars(null) targets the primary profile's env store,
+    // so a non-default profile's Keys page would read (and edit) the wrong
+    // profile. Unscoped must send undefined so the active profile applies.
+    await renderKeysSettings('tools')
+
+    await waitFor(() => expect(getEnvVars).toHaveBeenCalledWith(undefined))
+  })
+
   it('lists tools and excludes settings / channel-managed credentials', async () => {
     getEnvVars.mockResolvedValue({
       BRAVE_SEARCH_API_KEY: envVar('tool', { description: 'Search the web with Brave.' }),
diff --git a/apps/desktop/src/app/settings/keys-settings.tsx b/apps/desktop/src/app/settings/keys-settings.tsx
index aeb24b49be..c5f239cc6f 100644
--- a/apps/desktop/src/app/settings/keys-settings.tsx
+++ b/apps/desktop/src/app/settings/keys-settings.tsx
@@ -2,7 +2,7 @@ import { useStore } from '@nanostores/react'
 import { useCallback, useEffect, useMemo, useState } from 'react'
 
 import { useI18n } from '@/i18n'
-import { $settingsScopeOverride } from '@/store/settings-scope'
+import { $settingsRequestProfile } from '@/store/settings-scope'
 
 import { CredentialKeyCard, credentialPlaceholder, credentialRowLabel } from './credential-key-ui'
 import { useEnvCredentials } from './env-credentials'
@@ -35,8 +35,10 @@ const credentialElementId = (key: string) => `credential-key-${key}`
 export function KeysSettings({ view }: KeysSettingsProps) {
   const { t } = useI18n()
   // Shared settings "Applies to" scope: fetch + edit the selected profile's
-  // env store instead of the active one (null → active, the default path).
-  const scopeProfile = useStore($settingsScopeOverride)
+  // env store instead of the active one (undefined → active, the default
+  // path — request-shaped so the API helpers never see a primary-targeting
+  // null).
+  const scopeProfile = useStore($settingsRequestProfile)
   const { rowProps, vars } = useEnvCredentials(scopeProfile)
   const [openKey, setOpenKey] = useState<null | string>(null)
 
diff --git a/apps/desktop/src/app/settings/memory/connect.tsx b/apps/desktop/src/app/settings/memory/connect.tsx
index 64d9cfa6fa..1c093179fe 100644
--- a/apps/desktop/src/app/settings/memory/connect.tsx
+++ b/apps/desktop/src/app/settings/memory/connect.tsx
@@ -12,7 +12,7 @@ const POLL_TIMEOUT_MS = 120_000
 // Small connect affordance rendered under the provider dropdown. Capability is
 // backend-driven: the status route 404s for providers without an oauth_flow
 // module, so non-OAuth providers render nothing.
-export function MemoryConnect({ profile = null, provider }: { profile?: null | string; provider: string }) {
+export function MemoryConnect({ profile, provider }: { profile?: string; provider: string }) {
   const [capable, setCapable] = useState<'no' | 'unknown' | 'yes'>('unknown')
   const [connected, setConnected] = useState(false)
   const [auth, setAuth] = useState<MemoryProviderOAuthStatus['auth']>(null)
diff --git a/apps/desktop/src/app/settings/memory/provider-config-panel.tsx b/apps/desktop/src/app/settings/memory/provider-config-panel.tsx
index 2d7b6ae0c4..b4b5fcc1b2 100644
--- a/apps/desktop/src/app/settings/memory/provider-config-panel.tsx
+++ b/apps/desktop/src/app/settings/memory/provider-config-panel.tsx
@@ -20,7 +20,7 @@ function seedValues(config: MemoryProviderConfig): Record<string, string> {
   )
 }
 
-export function ProviderConfigPanel({ profile = null, provider }: { profile?: null | string; provider: string }) {
+export function ProviderConfigPanel({ profile, provider }: { profile?: string; provider: string }) {
   const [config, setConfig] = useState<MemoryProviderConfig | null>(null)
   const [loadError, setLoadError] = useState<null | string>(null)
   const [values, setValues] = useState<Record<string, string>>({})
diff --git a/apps/desktop/src/app/settings/model-settings.test.tsx b/apps/desktop/src/app/settings/model-settings.test.tsx
index 5343e2ef00..97d9eb84b4 100644
--- a/apps/desktop/src/app/settings/model-settings.test.tsx
+++ b/apps/desktop/src/app/settings/model-settings.test.tsx
@@ -28,11 +28,12 @@ const startManualProviderOAuth = vi.fn()
 let profileSwitchHandler: (() => void) | null = null
 
 vi.mock('@/hermes', () => ({
-  getGlobalModelInfo: () => getGlobalModelInfo(),
-  getGlobalModelOptions: () => getGlobalModelOptions(),
-  getAuxiliaryModels: () => getAuxiliaryModels(),
+  getGlobalModelInfo: (profile?: null | string) => getGlobalModelInfo(profile),
+  getGlobalModelOptions: (opts?: unknown, profile?: null | string) => getGlobalModelOptions(opts, profile),
+  getAuxiliaryModels: (profile?: null | string) => getAuxiliaryModels(profile),
   getApiRequestProfile: () => 'default',
-  getMoaModels: () => getMoaModels(),
+  getMoaModels: (profile?: null | string) => getMoaModels(profile),
+  profileScopeKey: (scope?: null | string) => (scope ?? '').trim() || 'default',
   setModelAssignment: (body: unknown) => setModelAssignment(body),
   getRecommendedDefaultModel: (slug: string) => getRecommendedDefaultModel(slug),
   saveMoaModels: (body: unknown) => saveMoaModels(body),
@@ -85,7 +86,7 @@ afterEach(() => {
   profileSwitchHandler = null
 })
 
-async function renderModelSettings() {
+async function renderModelSettings(scopeProfile?: string) {
   const { ModelSettings } = await import('./model-settings')
   const client = new QueryClient({ defaultOptions: { queries: { retry: false } } })
 
@@ -94,12 +95,36 @@ async function renderModelSettings() {
     // needs a router context in tests (the app provides HashRouter at root).
     <MemoryRouter>
       <QueryClientProvider client={client}>
-        <ModelSettings />
+        <ModelSettings scopeProfile={scopeProfile} />
       </QueryClientProvider>
     </MemoryRouter>
   )
 }
 
+describe('ModelSettings profile scope', () => {
+  // #90549: the API helpers treat `null` as "deliberately target the
+  // primary/default profile". A page following the active profile must pass
+  // `undefined`, or every read repaints the primary's model and the user's
+  // change looks reverted.
+  it('follows the active profile (undefined, never null) when unscoped', async () => {
+    await renderModelSettings()
+
+    await waitFor(() => expect(getGlobalModelInfo).toHaveBeenCalledWith(undefined))
+    expect(getGlobalModelOptions).toHaveBeenCalledWith(undefined, undefined)
+    expect(getAuxiliaryModels).toHaveBeenCalledWith(undefined)
+    expect(getMoaModels).toHaveBeenCalledWith(undefined)
+  })
+
+  it('reads through the explicit scope override when one is set', async () => {
+    await renderModelSettings('research')
+
+    await waitFor(() => expect(getGlobalModelInfo).toHaveBeenCalledWith('research'))
+    expect(getGlobalModelOptions).toHaveBeenCalledWith(undefined, 'research')
+    expect(getAuxiliaryModels).toHaveBeenCalledWith('research')
+    expect(getMoaModels).toHaveBeenCalledWith('research')
+  })
+})
+
 describe('ModelSettings', () => {
   it('loads the current main model and lists configured providers only', async () => {
     await renderModelSettings()
diff --git a/apps/desktop/src/app/settings/model-settings.tsx b/apps/desktop/src/app/settings/model-settings.tsx
index 3ad3af8ac2..6ae1590c8e 100644
--- a/apps/desktop/src/app/settings/model-settings.tsx
+++ b/apps/desktop/src/app/settings/model-settings.tsx
@@ -182,17 +182,15 @@ interface ModelSettingsProps {
   /** Notified after the main model is applied, so live UI stores can sync. */
   onMainModelChanged?: (provider: string, model: string) => void
   /** Shared settings "Applies to" scope: a concrete profile to edit instead of
-   *  the app's active one, or null to follow the active profile (default). */
-  scopeProfile?: null | string
+   *  the app's active one, or undefined to follow the active profile (default).
+   *  Request-shaped on purpose — the API helpers treat `null` as "deliberately
+   *  target the primary/default backend", so this prop never carries null. */
+  scopeProfile?: string
 }
 
-export function ModelSettings({ onMainModelChanged, scopeProfile = null }: ModelSettingsProps) {
+export function ModelSettings({ onMainModelChanged, scopeProfile }: ModelSettingsProps) {
   const { t } = useI18n()
   const m = t.settings.model
-  // `null` means "follow the active profile" at the settings layer. The
-  // Hermes API helpers use `undefined` for that contract; passing null would
-  // deliberately suppress the active profile and read the primary/default.
-  const requestProfile = scopeProfile ?? undefined
   const [loading, setLoading] = useState(true)
   const [error, setError] = useState('')
   const [mainModel, setMainModel] = useState<{ model: string; provider: string } | null>(null)
@@ -239,10 +237,10 @@ export function ModelSettings({ onMainModelChanged, scopeProfile = null }: Model
 
       try {
         const [modelInfo, modelOptions, auxiliaryModels, moaModels] = await Promise.all([
-          getGlobalModelInfo(requestProfile),
-          getGlobalModelOptions(undefined, requestProfile),
-          getAuxiliaryModels(requestProfile),
-          getMoaModels(requestProfile).catch(() => null)
+          getGlobalModelInfo(scopeProfile),
+          getGlobalModelOptions(undefined, scopeProfile),
+          getAuxiliaryModels(scopeProfile),
+          getMoaModels(scopeProfile).catch(() => null)
         ])
 
         if (profileEpoch.current !== epoch) {
@@ -280,7 +278,7 @@ export function ModelSettings({ onMainModelChanged, scopeProfile = null }: Model
         }
       }
     },
-    [requestProfile, scopeProfile]
+    [scopeProfile]
   )
 
   useEffect(() => {
@@ -543,7 +541,7 @@ export function ModelSettings({ onMainModelChanged, scopeProfile = null }: Model
       setConfig(next)
 
       try {
-        await saveHermesConfig(next, scopeProfile ?? undefined)
+        await saveHermesConfig(next, scopeProfile)
       } catch (err) {
         setConfig(prev)
         notifyError(err, m.defaultsFailed)
diff --git a/apps/desktop/src/store/settings-scope.test.ts b/apps/desktop/src/store/settings-scope.test.ts
index 85824a91c2..321ae5d31c 100644
--- a/apps/desktop/src/store/settings-scope.test.ts
+++ b/apps/desktop/src/store/settings-scope.test.ts
@@ -17,7 +17,7 @@ vi.mock('@/lib/query-client', () => ({ invalidateProfileScopedQueries: vi.fn() }
 vi.mock('@/store/starmap', () => ({ resetStarmapGraph: vi.fn() }))
 
 const { $activeGatewayProfile } = await import('./profile')
-const { $settingsScopeOverride, $settingsScopeProfile, setSettingsScope } = await import('./settings-scope')
+const { $settingsRequestProfile, $settingsScopeOverride, $settingsScopeProfile, setSettingsScope } = await import('./settings-scope')
 
 beforeEach(() => {
   $activeGatewayProfile.set('default')
@@ -58,6 +58,18 @@ describe('settings scope store', () => {
     expect($settingsScopeProfile.get()).toBe('default')
   })
 
+  it('exposes a request-shaped scope: undefined (never null) without an override', () => {
+    // api/client.ts profileScoped() treats null as "target primary/default" —
+    // the #90549 bug class. The request form must therefore never be null.
+    expect($settingsRequestProfile.get()).toBeUndefined()
+
+    setSettingsScope('research')
+    expect($settingsRequestProfile.get()).toBe('research')
+
+    setSettingsScope('default')
+    expect($settingsRequestProfile.get()).toBeUndefined()
+  })
+
   it('drops the override on an app-wide profile switch', () => {
     setSettingsScope('research')
     expect($settingsScopeOverride.get()).toBe('research')
diff --git a/apps/desktop/src/store/settings-scope.ts b/apps/desktop/src/store/settings-scope.ts
index c8178f22df..115707b8d5 100644
--- a/apps/desktop/src/store/settings-scope.ts
+++ b/apps/desktop/src/store/settings-scope.ts
@@ -16,6 +16,23 @@ export const $settingsScopeProfile = computed([$settingsScopeOverride, $activeGa
   normalizeProfileKey(override ?? active)
 )
 
+// ── Request-scope form (THE value to hand to API helpers) ──────────────────
+// The store contract and the API contract disagree about `null`:
+//   - here, `null` means "follow the app's active profile" (no override);
+//   - in api/client.ts `profileScoped()`/`capabilityScoped()`, `null` means
+//     "deliberately suppress the active profile and target primary/default" —
+//     only `undefined` falls back to the active profile.
+// Passing the raw override into an API helper therefore silently retargets
+// every read/write to the primary profile whenever no override is set — the
+// "model change reverts when I re-enter the tab" class of bug (#90549: the
+// page WROTE the right profile but READ primary back). Always send this
+// computed (or `override ?? undefined`) on requests; keep the raw override
+// only for UI concerns (selector highlight, cache keys, remount keys).
+export const $settingsRequestProfile = computed(
+  $settingsScopeOverride,
+  (override): string | undefined => override ?? undefined
+)
+
 // Select the profile the settings pages should edit. Picking the app's active
 // profile stores `null` (no override) so the scope keeps following the app on
 // profile switches — and requests keep their unscoped default shape.